In the processing of molten aluminum, it is desirable to remove certain gases and other material or elements from the molten aluminum before further processing, and depending upon the specific application or process. The equipment or function may generally be referred to as a degasser or degassing.
In a typical application of a degasser for molten aluminum, dissolved hydrogen from any one or more of multiple potential sources, is a targeted gas to be removed from the melt prior to the next step in the process (such as casting for instance). If for instance hydrogen remains in the aluminum during casting, hydrogen coming out of solution may cause any one or more of cast problems, such as twisting, flaking, blisters or even cracking. It is typically desirable to remove the dissolved hydrogen just prior to the next step in the process.
The particular dissolved hydrogen content in a given application may vary substantially, but can range from 0.20 ml/100 g Al for general extrusion billet down to 0.10 ml/100 g Al for rolling slab for aerospace types of applications.
Typically hydrogen is removed from the molten aluminum by introducing or bubbling an inert gas through the metal. Examples of inert gases which may be utilized include argon or nitrogen.
In addition to the removal of the hydrogen through the utilization of inert gases, it is also typical to desire to remove other impurities and/or inclusions during the refining process, and this removal may also occur or be desired during this degassing process. For instance, the addition of smaller amounts of chlorine in the inert gas may remove different inclusions and alkali metal impurities in a relatively efficient way. Inclusions in molten aluminum may come from any one or more different sources during the smelting operation, in the molten metal furnace or from intentionally added material such as grain refiners. The failure to adequately remove inclusions may result in tears and surface defects in rolling sheet aluminum, pinholes and increased die wear during extrusion. It is typical in some applications to target the removal of approximately 50% of non-wetted inclusions in the degassing system. Later filtering of the molten aluminum downstream from the degassing system would typically be utilized to further reduce inclusions in the molten metal.
A typical degasser system, or molten aluminum refining system for the removal of gases which utilizes a rotor within a stator, would typically involve the injection of an inert gas utilizing one or more injectors or injection devices, such as a spinning rotor device. The injector would typically introduce the inert gas, such as Argon, into the molten metal through numerous bubbles that the injector may shear and disperse into the molten metal in order to saturate the molten metal with the inert gas. In systems which do not use a stator, gases may be injected through the center of the rotating rotor shaft—however in many applications it is desired or preferred to utilize a stator for process and other reasons.
The inert gas is typically introduced into the molten metal near the bottom of the containment vessel and the bubbles of gas are dispersed and allowed to rise to the melt surface, desorbing the dissolved hydrogen in the process. The addition of chlorine as mentioned above in small amounts (such as 0.5% or less) may assist in breaking the bond between the molten aluminum and any non-wetted inclusions in the molten aluminum, thereby allowing the inclusions to more readily attach to the rising gas bubbles and be buoyed or lifted to the melt surface of the molten aluminum. Additional amounts of chlorine may be added to the inert gas to chemically react with incoming alkali metals such as sodium, lithium, calcium, or others, to form chloride salts that also float to the surface or melt surface of the molten aluminum.
Typically the inclusions and solid salts and other material that float to the melt surface form what is referred to as dross, which can then be skimmed from the surface and removed as waste.
It is typically desirable to maximize the saturation of the molten aluminum with small gas bubbles and to maintain a flat or calm melt surface to better facilitate the floating and capturing of inclusions and salts to the melt surface. Achieving these objectives will generally result in better separation of the molten aluminum from the dross. There are many factors that contribute to the efficiency of these systems, such as the nozzle or injector design, gas flow rates, the flatness of the molten aluminum melt surface, vessel chambered geometries, and others.
Some prior art injectors utilize a spinning rotor within a static stator to strive toward the desired saturation level, with the spinning rotor being attached or integral with a nozzle portion. The spinning rotor may actually be used to shear and help disperse the gas bubbles and any additions thereto, into the molten aluminum. It is also desirable, in order to maintain the melt surface relatively still or flat, to avoid a vortex effect from the rotation of the rotor. A vortex affect would tend to cause disruptions in the surface, a partially mixing or dispersion of the material in the dross with the molten aluminum, and generally interfere with or hinder the removal of undesirable gas and inclusions.
One example of a molten aluminum degassing or metal refining system is one offered by Pyrotek under the SNIF trademark. References and information relative to the Pyrotek products may be found at its website at www.pyrotek-inc.com.
Prior United States patents referring to such prior art systems, include the following: U.S. Pat. No. 5,198,180, for a Gas Dispersion Apparatus with a Rotor and Stator for Molten Aluminum Refining; U.S. Pat. No. 5,846,481, for a Molten Aluminum Refining Apparatus; U.S. Pat. No. 3,743,263, for an Apparatus for Refining Molten Aluminum; and U.S. Pat. No. 4,203,581, for an Apparatus for Refining Molten Aluminum; all of which are hereby incorporated in their entirety by this reference as those set forth fully herein.
In a typical prior art configuration for molten aluminum refining, one or more injectors such as injector 130 in FIG. 2, would be located within the molten aluminum or molten metal, and the gases would be introduced through that injector as described below.
It is also desirable to reduce the dissolved gas content and the non-metallic in purity content of the molten aluminum, and this is typically accomplished by utilizing any one or more of various fluxing processes, which is where the molten metal is contacted with either reactive gaseous or solid fluxing agents (such as halogens). Chlorine gas for instance may be utilized in the removal of the non-metallic impurities. If it is desired in a given application to also introduce flux into the molten aluminum, a separate piece of equipment, namely a device such as a flux injector, is introduced into the molten metal and flux is thereby delivered or injected into the molten aluminum. This requires an additional expense, additional capital outlay for the machinery, and additional maintenance thereon.
It is therefore an objective of embodiments of this invention to provide a molten aluminum refining system which will allow for the injection of gas and flux while utilizing a spinning rotor within a static stator.
While the invention was motivated in addressing some objectives, it is in no way so limited. The invention is only limited by the accompanying claims as literally worded, without interpretative or other limiting reference to the specification, and in accordance with the Doctrine of Equivalents. Other objects, features, and advantages of this invention will appear from the specification, claims, and accompanying drawings which form a part hereof. In carrying out the objectives of this invention, it is to be understood that its essential features are susceptible to change in design and structural arrangement, with only one practical and preferred embodiment being illustrated in the accompanying drawings, as required.